The manufacture of many types of work pieces requires the substantial planarization of at least one surface of the work piece. Examples of such work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. One commonly used technique for planarizing the surface of a work piece is the chemical mechanical polishing (CMP) process, a process commonly practiced in a multi-zonal processing apparatus. In the CMP process a work piece, held by a work piece carrier head, is pressed against a polishing pad and relative motion is initiated between the work piece and the polishing pad in the presence of a polishing slurry. The mechanical abrasion of the surface combined with the chemical interaction of the slurry with the material on the work piece surface ideally produces a surface of a desired shape, usually a planar surface. The terms “planarization” and “polishing,” or other forms of these words, although having different connotations, are often used interchangeably by those of skill in the art with the intended meaning conveyed by the context in which the term is used. For ease of description such common usage will be followed and the term “chemical mechanical polishing” will generally be used herein with that term and “CMP” conveying either “chemical mechanical planarization” or “chemical mechanical polishing.” The terms “planarize” and “polish” will also be used interchangeably.
The construction of the carrier head of a CMP apparatus and the relative motion between the polishing pad and the carrier head as well as other process variables have been extensively engineered in an attempt to achieve a desired rate of removal of material across the surface of the work piece and hence to achieve the desired surface shape. For example, the carrier head generally includes a flexible membrane that contacts the back or unpolished surface of the work piece and accommodates variations in that surface. A number of pressure chambers are provided behind the membrane so that different pressures can be applied to various zones on the back surface of the work piece to cause desired variations in polishing rate across the front surface of the work piece.
End point detection probes are often used to detect the completion of a polishing operation. The completion of the polishing operation is signaled, in accordance with a detection algorithm, as a function of the remaining material thickness. Upon detection of the end point signal, the CMP operation is either terminated immediately or after some prescribed delay denoted as an “over polish time.” In order to increase the detection coverage area on the work piece, a plurality of end point detection probes can be used. When using a plurality of probes, the CMP operation is terminated after end point detection signals are received from all of the probes. The use of a multi-zone carrier head in conjunction with a plurality of end point detection probes can improve CMP results if, upon receipt of a signal from one of the end point detection probes, the pressure in one or more of the particular zones of the carrier head is reduced, thereby locally reducing the polishing pressure. This approach, however, has a number of deficiencies. For example, some of the zones in the carrier head may be pressurized to their full pressure while an adjacent zone is at zero pressure. The severe pressure gradient between zones creates a significant stress on the surface of the work piece being polished and can damage structures on the work piece surface. In addition, the relative motion between the carrier head and the polishing pad is intentionally randomized to aid in achieving the desired polishing profile across a work piece. Because of the randomized motion, there is no direct correlation between the area on the work piece surface being monitored by a particular end point detection probe and the area controlled by a specific zone of the carrier head.
In many applications of chemical mechanical polishing, it is desirable to serially process a large number of work pieces, each of which may have similar surface characteristics. For example, in the semiconductor industry lots of twenty or more semiconductor wafers may be serially processed through a given CMP apparatus. Each of the wafers in the lot will be in a similar process state. For example, each of the wafers in the lot may have just had a layer of material such as layer of copper or other material deposited on one surface. A single piece of deposition equipment will have been used to deposit the layer on each of the wafers. The layer will have relatively uniform characteristics, such as thickness and deposition profile, from wafer to wafer, and those characteristics will be a function of the particular deposition equipment.
The CMP operation ideally achieves the desired shape across an individual work piece and from work piece to work piece within a lot. The CMP processing of work pieces can be a slow process, especially because the work pieces must be processed individually rather than in batches. To achieve a high throughput for the CMP operation, with desired processing results, a method is required that provides reliable run to run controls.
Accordingly, it is desirable to provide a method for controlling a CMP operation. In addition, it is desirable to provide a method for controlling the process variables in a multi-variable CMP operation, and especially to provide a method for controlling a CMP operation from run to run, that is, from work piece to work piece. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.